Update ADK doc according to issue #1587 - 13

docs/integrations/secret-manager.md

@@ -0,0 +1,78 @@
+---
+sidebar_label: Google Cloud Secret Manager
+title: Google Cloud Secret Manager Integration
+---
+
+The ADK includes a built-in client for Google Cloud Secret Manager that simplifies retrieving secrets. This allows you to securely store and access sensitive information like API keys or other credentials required by your ADK tools.
+
+The primary class for this integration is `SecretManagerClient`, which can be imported from `google.adk.integrations.secret_manager.secret_client`.
+
+## Instantiating the Client
+
+You can instantiate the `SecretManagerClient` in a few different ways, depending on your authentication needs.
+
+### Using Application Default Credentials (ADC)
+
+If your application is running in a Google Cloud environment (like Cloud Run or GKE) or you have configured the gcloud CLI locally with `gcloud auth application-default login`, you can instantiate the client without any arguments. It will automatically find the necessary credentials.
+
+```python
+from google.adk.integrations.secret_manager import secret_client
+
+client = secret_client.SecretManagerClient()
+```
+
+### Using a Service Account
+
+You can authenticate by passing the contents of a service account JSON key file as a string.
+
+```python
+from google.adk.integrations.secret_manager import secret_client
+import json
+
+# Assume 'service_account_dict' is a dictionary containing your service account key
+service_account_json_string = json.dumps(service_account_dict)
+
+client = secret_client.SecretManagerClient(
+    service_account_json=service_account_json_string
+)
+```
+
+### Using an OAuth 2.0 Access Token
+
+If you have a pre-existing OAuth 2.0 access token, you can use it to instantiate the client.
+
+```python
+from google.adk.integrations.secret_manager import secret_client
+
+auth_token = "your-oauth2-access-token"
+
+client = secret_client.SecretManagerClient(auth_token=auth_token)
+```
+
+## Retrieving a Secret
+
+Once the client is instantiated, you can retrieve a secret by calling the `get_secret()` method. This method requires the full resource name of the secret version you wish to access.
+
+The resource name has the following format:
+`projects/your-gcp-project-id/secrets/your-secret-name/versions/your-secret-version`
+
+Typically, you will want to retrieve the latest version of a secret, which can be specified using `latest`.
+
+```python
+# Assuming 'client' is an instantiated SecretManagerClient
+resource_name = "projects/my-gcp-project/secrets/api-key/versions/latest"
+
+try:
+    secret_value = client.get_secret(resource_name)
+    print(f"Successfully retrieved secret: {secret_value}")
+except Exception as e:
+    print(f"Error retrieving secret: {e}")
+
+# You can now use the secret_value with other ADK tools
+# For example, passing it to a model's API key parameter
+#
+# from google.adk.models import some_model
+#
+# model = some_model.SomeModel(api_key=secret_value)
+
+```